Synthesis of late Paleozoic and Mesozoic eolian deposits of the ...

erg system toward the Utah-Idaho trough, and an
eastward thinning of the system across the craton
to the irregular pinch-out along a north-south line
in eastern Colorado. Superimposed on this overall
trend, the isopach patterns reflect tectonic ele-
ments that were active at the time of deposition
(Fig. 20).
Extending eastward at high angles to the trend
of the Utah-Idaho trough are elongate troughs
and basins. These include the Uinta Mountain
trough, ancestral Uinta basin, an unnamed basin,
the Circle Cliffs trough and the Black Mesa trough.
Additional basins are evident eastward on the
craton. These include the San Juan basin and a
basin in east-central New Mexico.
Areas of relative uplift marked by thinning
strata are also evident. Most evident are the ele-
ments of the Ancestral Rockies (Front Range and
Uncompahgre positive areas) where isopach pat-
terns show a thinning toward these elements. An
unnamed high bisects the San Juan basin. The
Defiance uplift, Monument bench, and an un-
named high in central Utah are evident from
thinning of strata over them.
Facies patterns within the main body of the
Entrada system are complex (Fig. 27). In general,
there is a westward decrease in the percent of
cross-stratified sandstone. Strata consisting of over
75% eolian cross-strata occur east of the
Utah-Colorado border and east of the San Juan
basin in southwestern Colorado and New Mexico.
Similarly high percentages of cross-stratified sand-
stones occur near the outcrop limits in west-central
New Mexico, southernmost central Utah and
north-central Arizona. The general trend toward
less eolian cross-strata westward is also disrupted
by the presence of silty sandstone (beds at Baby
Rocks) west of the Defiance uplift. Low per-
centages of eolian cross-strata occur in the south-
eastern part of the map area within the margins of
the unit but near its pinch-out, and adjacent to
elements of the Ancestral Rockies. Adjacent to the
western portion of the eolian system are large
quantities of mudstone. Here the Entrada Sand-
stone grades laterally into the Preuss Formation.
Aspects of depositional environments of the
Entrada Sandstone have been studied in part
(Kocurek, 1981; VinceUet and Chittum, 1981;
Kocurek and Dott, 1983; Rubin and Hunter, 1983;
Lucas et al., 1985; Peterson, 1986; Condon and
Peterson, 1986), but, as with most of the eolian
units of the western United States, a detailed,
regional synthesis has not yet been attempted.
Complex facies changes and vertical and lateral
differences in the style of cross-strata occur across
the region. In a broad view, however, regional
patterns are evident. The main body of the En-
trada consists of erg-sabkha-shallow marine de-
posits. Parts of the system represented primarily
by eolian dune deposits occur in Colorado and
eastern New Mexico; sandy, silty-sandy, sabkha-
shallow marine deposits characterize the western-
most portions of the system. Between these facies
are units characterized by alternating cross-strati-
fied and flat-bedded deposits. Flat-bedded de-
posits almost certainly represent thick interdune
deposits and inland sabkhas. Areas also marked
by the accumulation of primarily dune cross-strata
occur near the southern limits of the outcrop in
north-central Arizona and western New Mexico.
These outcrops mark a southern, inland erg that
extended an unknown distance southward toward
the volcanic arc. A possible inland sabkha deposit
extends around the north and east flanks of the
Defiance uplift and is represented by the beds at
Baby Rocks. Flat-bedded units present in areas
adjacent to the Ancestral Rockies and on the High
Plains probably represent sand-sheet and alluvial
systems. Judging by the thinning of strata toward
the Ancestral Rockies and the facies change from
dune to sand-sheet deposits, parts of the Front
range and Uncompahgre Plateau formed positive
features during Entrada time.
Middle Jurassic IV
Figure 28 and accompanying cross-sections
(Fig. 29C, D, E, F) illustrate the thickness and
distribution of the upper member of the Entrada
Sandstone in west-central Utah, the Cow Springs
Member of the Entrada Sandstone in Arizona and
New Mexico, and the sandstone at Mesita in New
Mexico.
The upper member of the Entrada Sandstone
represents an eolian sandstone depocenter that
overlies largely sabkha deposits of the middle
member of the Entrada Sandstone. The upper
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